Table of contents

This special issue of Nanotechnology contains selected papers presented at the fourth `Trends in Nanotechnology' (TNT2003) international conference, held in Salamanca (Spain), 15-19 September 2003.

In response to the growing awareness of the importance of nanotechnology, many conferences are being organized worldwide to discuss the latest advances. Among these, the conference series `Trends in Nanotechnology' (Toledo, Spain, 2000; Segovia, Spain, 2001; Santiago de Compostela, Spain, 2002) has become a key meeting in the nanotechnology field. It provides fresh ideas, brings together well-known speakers, and promotes a suitable environment for discussions, exchanging ideas, and enhancing scientific and personal relations among participants. TNT2003 was organized in a similar way to the three prior TNT conferences, in large part due to their overwhelming success. In 2003, more than 360 scientists from Europe, the United States, Japan and other countries attended this meeting and contributed with talks (76), posters (260) and stimulating discussions about their most recent research.

The aim of the conference was to focus on the applications of nanotechnology and to bring together, in a scientific forum, various worldwide groups belonging to industry and public institutions. TNT2003 was particularly effective at transmitting information and establishing contacts among workers in this field. Graduate students attending such conferences quickly learn the importance of interdisciplinary skills and become more effective in their future lines of research. Last year, almost 65 graduate students received a grant (from NASA, ONRIFO, PHANTOMS or TNT) to attend the conference and present their work through a poster (16 prizes to the best posters were awarded during this event). The scientific programme, without parallel sessions, covered a wide spectrum of nanotechnology research.

We would like to thank all the participants for their assistance as well as the authors for their written contributions. We are indebted to the following institutions, companies and government agencies for their help and financial support: Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, CMP Cientifica, University of Cambridge, University of Purdue, Universidad Complutense de Madrid, Universidad Carlos III de Madrid, Universidad de Salamanca, Universidad de Santiago de Compostela, Georgia Institute of Technology, Universidad SEK, IRC in Nanotechnology, PHANTOMS Network (EU funded through the IST programme), ONRIFO, NASA, NIMS (Nanomaterials Laboratory), Nanotechnology Research Institute, Fundetel, Parque Cientifico de Madrid, Ayuntamiento de Salamanca, Caja Duero, Air Force Office of Scientific Research, Motorola, World Scientific, WILEY-VCH, Institute of Physics Publishing and the Ministerio Español de Ciencia y Tecnología. We would also like to thank the following companies for their participation as exhibitors: NanoTec, Raith, Orsay Physics, SPECS, World Scientific and Institute of Physics Publishing.

We invite readers of this special issue of Nanotechnology to join us at the next `Trends in Nanotechnology' conference. TNT2004 will be held in Segovia (Spain), 13-17 September 2004.

In Stadler et al (2003 Nanotechnology 14 138), a scheme for a molecular memory was presented. It was based on the influence of the positions of chemical side-groups attached to aromatic molecules on the paths for electrons propagating through these molecules in the ballistic and tunnelling transport regimes. Here we extend this concept in the following ways. (i) A graphical method is derived from an electron scattering formalism based on a topological Hückel description, which allows us to estimate whether the electron transport between two electrodes attached to specific atomic sites in an arbitrary molecule is finite or zero at the Fermi level. (ii) The same scheme that was used for the implementation of the molecular memory is extended to logic functions, in particular a half-adder. (iii) A more realistic description of the chemical nature of the proposed intra-molecular circuits is achieved by using the elastic scattering quantum chemistry (ESQC) technique in an extended Hückel implementation and by specifying the side-groups as nitro-groups, which are rotated in order to feed the signal inputs into the computational circuit.

SrZrO₃ thin films grown by pulsed laser deposition on SrRuO₃-buffered SrTiO₃ substrates have been investigated by means of x-ray diffraction, atomic force microscopy and Rutherford backscattering spectroscopy. The compressive stress originated in the coherent SrZrO₃/SrRuO₃ interface due to the lattice mismatch () forces epitaxial SrZrO₃ to grow in a three-dimensional habit. So-deposited SrZrO₃ is used to fabricate nanoarrays of dielectric wires from a nanopatterned SrRuO₃ surface. The influence of the geometric shadowing effect produced by the non-perpendicular incidence of the ablated particles on the SrZrO₃ growth habit is discussed.

Stacked layers of In(Ga)As on GaAs(001) self-assembled quantum rings (QR) for laser application have been studied. Several samples with three stacked QR layers have been grown by molecular beam epitaxy with GaAs spacers from 1.5 to 14 nm. The optical and structural properties have been characterized by photoluminescence spectroscopy and by atomic force microscopy, respectively. For GaAs spacers larger that 6 nm, the stacked QR layers present similar properties to single QR layers. A semiconductor laser structure with three stacked layers of QR separated 10 nm in the active region has been grown. This spacer ensures well-developed rings with optical emission like that of a single layer. Laser diodes have been processed with 1–2 mm cavity lengths. The stimulated emission is multimodal, centred at 930 nm (77 K), with a threshold current density per QR layer of 69 A cm⁻². In this work, it is demonstrated that stacking rings is possible, and that a broad area laser with three QR layers can be fabricated successfully.

Diluted arrays of antidots have been patterned by electron beam lithography and an etching process on amorphous Co–Si films of well defined uniaxial anisotropy. The analysis of the angular dependence of the hysteresis loops shows that the antidot arrays present a similar uniaxial anisotropy to the unpatterned film, and that the main effect of patterning for this small antidot density appears as an enhancement in the coercivity. The observed easy axis coercive fields are consistent with the estimates for domain wall pinning by a non-magnetic inclusion surrounded by a closure domain structure. However, the angular dependence of the coercivity presents an anomalous behaviour that points to the existence of an anisotropic domain wall pinning mechanism of the antidot arrays.

We observed the dynamic behaviour of a single conjugated molecule, 4-biphenylmethanethiol (BPMT), inserted in a self-assembled monolayer (SAM) matrix of a bicyclo[2.2.2]octane derivative (BCO) using scanning tunnelling microscopy at . At room temperature (Wakamatsu et al 2003 Nanotechnology14 258), stochastic switching of BPNT and lateral movement of the BPMT and the BCO molecules were observed even in the well-ordered BCO SAM matrix on Au(111) terraces. Such movement was, however, rarely observed in the SAM on Au(111) terraces at . Single BPMT molecules were fixed in the well-ordered BCO SAM matrix at , except at step edges and edges of one atom deep pits of the Au(111) substrate. When a disordered BCO SAM matrix was created intentionally by applying voltage pulses, the stochastic switching of the single BPMT molecule and the lateral movement of the BCO and the BPMT molecules were observed more frequently than those at the step edges and the pit edges at .

Electronic properties of proposed new families of carbon single walled nanotubes are investigated. These nanotubes, called graphynes, result from the elongation of covalent interconnections of graphite-based nanotubes by the introduction of yne groups. Analogous to ordinary nanotubes, armchair, zigzag and chiral graphyne nanotubes are possible. Tight-binding and ab initio density functional methods were used to predict the electronic properties of these unusual nanotubes. Of the three graphyne nanotube families analysed here, two provide metallic behaviour for armchair tubes and either metallic or semiconducting behaviour for zigzag nanotubes. For the other graphyne nanotube family investigated a diameter and chirality independent bandgap is predicted and a bandgap modulation study by structural distortions has been carried out for small longitudinal tube deformations. Interestingly, while the bandgap is insensitive to structure, the stress-induced bandgap changes can strongly depend both on the nanotube type and whether the strain is tensile or compressive.

Self-assembled monolayers (SAMs) of thiolates with a cycloalkane or a bicycloalkane moiety were formed on Au(111). The structure of the SAMs was characterized using scanning tunnelling microscopy (STM) and compared with that of linear alkane thiolates. In addition, their electrical properties in terms of dependence of tunnelling currents on the number of the alkane chains in the alicyclic moieties were investigated. For this purpose, we prepared mixed SAMs, in which two of three kinds of thiolate were molecularly mixed. By comparing STM height differences between the two thiolates in the mixed SAMs in a constant-current mode, the charge transport properties were investigated.

Colloidal suspensions of iron oxide and metal iron nanoparticles prepared by laser pyrolysis have been obtained by coating the particles with dextran in an aqueous media giving rise to biocompatible ferrofluids. The structural characteristics of the powders and the size of the particles and the aggregates in the colloidal suspensions have been analysed and correlated with the magnetic properties of both solids and fluids. For the first time, to our knowledge, a stable ferrofluid based on metal particles (<10 nm) has been obtained with aggregate sizes of  nm. In comparison to iron oxide based products, this material exhibits higher saturation magnetization (45 emu g⁻¹) and susceptibilities (4000 emu/g T). In addition, the nuclear magnetic resonance response of the ferrofluids has been measured in order to gain information about the influence of the crystallochemical and magnetic properties on their relaxation behaviour. The main parameter affected by the presence of the magnetic nanoparticles is the transversal relaxation time T₂ and the corresponding relaxivity R₂ value that is of the order of 400 (mmol/l)⁻¹ s⁻¹. It has been shown that R₂ value increases not only by using iron metal instead of iron oxide but also by increasing the crystal size of the particles. From this study an evaluation of the possibilities of these materials as contrast agents for magnetic resonance imaging has been made.

Magnetic MnZn-ferrite nanoparticles with a narrow size distribution were prepared in water–CTAB–hexanol microemulsions. The region of microemulsion stability in the system was determined, using the titration method, as a function of the temperature and of the type and concentration of solutes in the aqueous phase. The nanoparticles were prepared in a two-step process: the precipitation of the corresponding hydroxides, followed by oxidation of the Fe²⁺. The particle size was controlled by the composition of the microemulsion and the concentration of the reactants (the corresponding sulfates and a precipitation agent, tetramethyl ammonium hydroxide) in the aqueous solution of the microemulsion. The specific magnetization of the nanoparticles (measured at 13 kOe) was found to depend mainly on particle size: ranging from 1.3 emu g⁻¹ for particles of approximately 2 nm in size to 7.3 emu g⁻¹ for particles of approximately 5 nm in size.

The puzzling non-universal conductance quantization in two-terminal fully-ballistic long quantum wires is explained, within a many-particle wavepacket formalism, as a limitation of the electron injecting flux due to the fermion exchange-interaction effect. The Landauer conductance step is obtained when injection of electrons, one by one, is considered. Reductions of G₀ in long quantum wires are due to a many-particle exchange interaction effect when several electrons are injected, simultaneously, into the system. Numerical results are provided for conductance step reductions in ballistic GaAs/AlGaAs quantum wires longer than 1 µm that are in good agreement with experimental results.

Tapping mode atomic force microscopy (TM-AFM) in an ambient environment is a widely employed tool in the field of characterization of materials at the nanoscale. Significant advances have recently been made in the understanding of the physics behind some of the complexities of its operation, the most profound being the prediction and demonstration of the existence of the attractive and repulsive regimes of tip–sample interaction. In this paper we present an investigation of the criteria required for accessing the two imaging regimes, a simple method for controlling the transition between them in situ, and an assessment of their consequences for topographic and phase shift images of DNA. We find that the transition from repulsive to attractive regime imaging is characterized by a large increase in topographic height and concomitant decrease and sign inversion of the phase shift recorded over single molecules of DNA on mica. By varying the frequency at which the cantilever is driven, we can select which regime we wish to operate in routinely and reproducibly. Controlling the tip–sample interaction in this way greatly improves images of fragile nanoscale structures such as single molecules.

We report molecular dynamics studies of carbon nanotubes as mechanical gigahertz oscillators. Our results show that different oscillatory regimes exist but that sustained oscillations are possible only when the radii difference values of the inner and outer tubes are . Frequencies as large as 87 GHz were obtained. Calculated force and frequency values are in good agreement with estimated data from recent experimental investigations.

We have obtained Al(III)-protected Fe–Co metal nanoparticles with acicular shape by thermal reduction with hydrogen from Al(III)–Co(II)-codoped acicular goethite precursors prepared by oxidation with air of a FeSO₄ solution containing Al(NO₃)₃ and Co(NO₃)₃ precipitated with Na₂CO₃. These precursor particles were smaller ( nm in length) than those prepared by a previously reported procedure, resulting in smaller metal particles ( nm in length) suitable for high-density magnetic recording media in which higher bit-packing densities could be obtained. The location of Co and Al in the goethite precursors, as well as in the final metal particles, have been studied for a better understanding of role that these elements play in the microstructural features and the magnetic properties of the final metal particles. It was found that the Al content in the particle outer layers was enhanced during the reduction process, while cobalt diffuses toward the inner part of iron nanoparticles forming an Fe–Co alloy. The incorporation of cobalt helps to increase the magnetization saturation because it avoids corrosion and minimizes the growth of the iron crystals. The presence of Al(III) in the particle outer layers of the precursors inhibited the growth of iron crystals and preserved the acicular shape during the reduction process, which has a very favourable effect on the coercivity and the squareness of the metal samples.

Nanostructuring of metallic and semiconductor surfaces in the sub-100 nm range is a key point in the development of future technologies. In this work we describe a simple and low-cost method for metal nanostructuring with 50 nm lateral and 6 nm vertical resolutions based on metal film deposition on a silane-derivatized nanostructured silicon master. The silane monolayer anti-sticking properties allow nanopattern transfer from the master to the deposited metal films as well as easy film detachment. The method is non-destructive, allowing the use of the derivatized master several times without damaging. Potential applications of the method are in the field of high-density data storage, heterogeneous catalysis and electrocatalysis, microanalysis (sensors and biosensors) and new optical devices.

Bismuth nanowires with diameters between 80 and 500 nm are electrochemically fabricated in etched single-pore membranes. By means of two macroscopic planar electrodes on either side of the membrane, single nanowires are successfully contacted with contact resistances of only several ohms. The contacting method is presented and the measured resistance-versus-diameter behaviour of Bi nanowires is discussed, considering both electron scattering at the wire surface and that at the grain boundaries. Since the wires remain embedded in the membrane, lithographic processes and direct exposure of the wires to air are avoided.

In this work, n-type triple-gate metal–oxide–semiconductor field effect transistors (MOSFETs) are presented, where laser interference lithography (LIL) is integrated into a silicon-on-insulator (SOI) CMOS process to provide for the critical definition of the transistor channels. A mix and match process of optical contact lithography and LIL is developed to achieve device relevant structures. The triple-gate MOSFETs are electrically characterized to demonstrate the feasibility of this low cost fabrication process.

Powders consisting of ferromagnetic (FM) Fe nanoparticles, of about 7 nm in size, embedded in an antiferromagnetic (AFM) Cr₂O₃ matrix have been obtained by high-temperature reduction under a hydrogen atmosphere of a mixed Cr–Fe oxide. This FM–AFM system exhibits exchange bias effects, i.e. a loop shift (H_E) and coercivity enhancement (ΔH_C), when field-cooled through the Néel temperature, T_N, of Cr₂O₃. The exchange bias properties were measured as a function of temperature. H_E and ΔH_C are found to vanish at about T_N(Cr₂O₃), indicating a good quality AFM matrix. Hence, high-temperature reduction of mixed oxides is demonstrated to be a suitable technique to develop new types of FM–AFM exchange-biased nanoparticles, from which novel applications of this phenomenon may be developed.

Within the framework of the continuum elasticity theory, we have investigated the substrate orientation effects on the Stranski–Krastanov growth mode in Ge/Si heterostructures. To do this, we have estimated the transition thickness and accumulated stress in Ge/Si(111) low-dimensional systems and we have compared these with the values obtained previously for Ge/Si(001) heterostructures. The systems under investigation are pseudomorphic structures with a coherent behaviour at the substrate/film interface. For the dependence of the lattice parameter on the height, a sigmoidal-type function with appropriate constraints is considered. To evaluate the transition thickness, a minimization of the total free energy density with respect to the slope is made. Two different regimes for the accumulated stress are obtained in the range of investigated coverage. These regimes are directly related to the two stages of the Stranski–Krastanov mode. Although increasing Miller indices in the substrate induces a delay in the 2D–3D phase transition with a greater wetting layer, the relief strain mechanisms seem similar to those of Ge/Si(001) heterostructures. The non-rigid-substrate approximation applied in the Ge/Si(111) system also yields successful results for the transition thickness and compressive stress values, the fraction of strained substrate being roughly double in Ge/Si(111). In Ge_xSi_1−x/Si (111) heterostructures, the transition thickness is inversely proportional to the Ge composition and the compressive stresses are smaller for decreasing misfit strain. This feature is similar to that of the Ge_xSi_1−x/Si (001) system.

In nanoelectronics the search for a successor to CMOS technology has so far mainly been concentrated on nanodevices that could eventually be made much smaller than CMOS transistors, perhaps even brought down to the ultimate limit of the molecular scale. Much less effort has been invested in examining the architectural problems that will also have to be solved. These problems range from the device level up to the full circuit level, and they involve many different factors. The question of how much extra performance will be gained by going to new nanoscale devices is fundamentally linked to these architectural issues. This short review concentrates on recent work that has been done on architectures in nanoelectronics and includes topics such as the system dependent relation between performance and power dissipation, the applicability of concepts of parallelism and fault tolerance, and the design and fabrication of large circuits.

We demonstrate effective electron cooling of the normal metal strip by superconductor–insulator–normal metal (SIN) tunnel junctions. The improvement was achieved by two methods: first, by using an advanced geometry of the superconducting electrodes for more effective removal of the quasiparticles; and second, by adding a normal metal trap just near the cooling junctions. With simple cross geometry and without normal metal traps, the decrease in electron temperature is 56 mK. With the advanced geometry of the superconducting electrodes, the decrease in electron temperature is 129 mK. With the addition of the normal metal traps, the decrease in electron temperature is 197 mK.

We present and analyse solutions of a recent derivation of a drift-diffusion model of miniband transport in strongly coupled superlattices. The model is obtained from a single-miniband Boltzmann–Poisson transport equation with a BGK (Bhatnagar–Gross–Krook) collision term by means of a consistent Chapman–Enskog expansion. The reduced drift-diffusion equation is solved numerically and travelling field domains and current oscillations are obtained. A broad range of frequencies can be achieved, depending on the model parameters, in good agreement with available experiments on GaAs/AlAs superlattices.

Due to the nanometric properties of some physiological components of bone, nanomaterials have been proposed as the next generation of improved orthopaedic implant materials. Yet current efforts in the design of orthopaedic materials such as titanium (Ti) are not aimed at tailoring their nanoscale features, which is now believed to be one reason why Ti sometimes fails clinically as a bone implant material. Much effort is thus being dedicated to developing improved bioactive nanometric surfaces and nanomaterials for biospecificity. Helical rosette nanotubes (HRN) are a new class of self-assembled organic nanotubes possessing biologically-inspired nanoscale dimensions. Because of their chemical and structural similarity with naturally-occurring nanostructured constituent components in bone such as collagen and hydroxyapatite, we anticipated that an HRN-coated surface may simulate an environment that bone cells are accustomed to interacting with. The objective of the present in vitro study is therefore to determine the efficacy of HRN as a bone prosthetic material. Results of this study clearly show that both HRN-K1 and HRN-Arg coated Ti displayed enhanced cell adhesion when compared to uncoated Ti. Enhanced cell adhesion was observed even at concentrations as low as 0.005 mg ml⁻¹. These results point towards new possibilities in bone tissue engineering as they serve as a starting point for further mechanistic studies as well as future manipulation of the outer chemistries of HRN to improve the results beyond those presented here. One such effort is the incorporation of peptide sequences on the outer surface of HRN and/or growth factors known to enhance bone functions.

High quality luminescent CdS nanocrystals (NCs), with band edge emission tunable in the blue region of the visible spectrum, were synthesized by means of thermal decomposition of precursors with oleic acid as the surfactant and incorporated in optically transparent polystyrene (PS) and poly(methyl methacrylate) (PMMA). The optical properties of the obtained nanocomposite films were investigated by absorption and emission spectroscopies. Enhancement of the band edge luminescence of CdS quantum dots was observed in both polymer matrices and the fluorescence of the composites retained narrow emission profiles. Capping exchange at the NC surface with octylamine, demonstrated to improve the luminescence properties in solution, was performed in order to investigate the role played by surface ligands in the nanocomposite. Octylamine capped NCs immobilized in the polymers showed high defect state emission and visible quenching of band edge luminescence, indicating an effective interaction between oleic acid and PMMA and PS polymeric chains.

An eight-band Kane Hamiltonian modified for the strain has been used to describe the electronic states of the highly strained zincblende GaAs_1−xN_x. Conduction and valence band offsets of GaAs_1−xN_x with respect to InAs on InP(111) have also been investigated for different nitrogen (N) concentrations. A critical concentration of N is found, which marks the onset of type-I to type-II band alignment for GaAs_1−xN_x/InAs on InP(111). The effect of nitrogen on the conduction band of GaAs_1−xN_x has been described by the band anticrossing model. The strain balanced InAs /GaAs_1−xN_x short period superlattice on InP is predicted to reach operating wavelengths beyond .

We present a Monte Carlo study of an InGaAs based four-terminal ballistic rectifier operating at different temperatures. The rectifying effect is due to the vertical asymmetry of the electron concentration originated, in the presence of ballistic transport, by the action of an obstacle located in the centre of a ballistic cross junction. An increase of temperature degrades the efficiency of the device, since transport becomes more diffusive. However, it shows an intrinsic capability for rectification up to a frequency of 1.0 THz almost independently of the temperature.

Ellipsoidal metal nanoparticles about 200 nm in length and with different axial ratios were obtained by reduction with hydrogen of an iron oxide. These metal particles were stabilized without the presence of an antisintering and protecting layer of aluminium or yttrium oxide, giving rise to a significant improvement of the magnetic properties. The precursors were uniform ellipsoidal haematite particles synthesized by forced hydrolysis of iron perchlorate in the presence of urea and phosphate ions. A detailed characterization of the nanoparticles was carried out to correlate the microstructure of the haematite precursors with the structural and magnetic properties of the final metal particles. It was observed that the single-crystal character of the particles is preserved during the transformation of iron oxide to metal. The resulting metal particles consist of a metal core of α-Fe and an oxide layer about 5 nm thick, with a spinel structure. The magnetic properties of this material showed very high saturation magnetization () and coercivity values increasing from 1000 to 1200 Oe as the particle axial ratio increases. Measurements of the time dependence of the magnetization yielded activation volumes eight and five times smaller than the particle physical volumes, suggesting a mechanism of incoherent reversal of the magnetization.

The interacting induced dipole polarization model is used for the calculation of the dipole–dipole polarizability α. The method is tested with single-wall carbon nanotubes (SWNTs) as a function of nanotube radius and elliptical deformation. The results are similar to ab initio reference calculations. For the zigzag tubes, the polarizability follows a remarkably simple law. The calculations effectively differentiate among SWNTs with increasing radial deformations. The polarizability and related properties can be modified continuously and reversibly by the external radial deformation. These results suggest a technology in which mechanical deformation can control chemical properties of the carbon nanotubes. Different effective polarizabilities are calculated for the atoms at the highest and lowest curvature sites. The calculations efficiently differentiate between the effective polarizabilities of the highest and lowest curvature sites. MOPAC-AM1 heat of formation per C atom shows that SWNT hydrocarbons (SWNTHCs) are less stable than planar acenes. SWNTHCs are stabilized and acenes are destabilized with increasing number of vertices. For SWNTs, the ratio of trivalent/divalent vertices is greater than that for the corresponding planar acenes.

In this paper we report on thin film transistors based on drop casting solutions of regioregular poly(3-hexylthiophene) (P3HT) over prefabricated gold electrodes. This polymer is known to self-organize into a lamellar structure in chloroform resulting in high field-effect mobilities. We studied the dependency of the charge carrier mobility of devices prepared from solution in chloroform with electrode spacings ranging from 5 µm to 20 nm. It was found that the overall trend was that the mobility decreased as the electrode spacing was made smaller, indicating that the transport properties on closely spaced electrodes were dominated by the contacts. Applying an ac voltage during the preparation of such films resulted in lower mobilities. However, P3HT in p-xylene forms fibres, which were aligned between the electrodes by applying an ac field. Films of aligned fibres with mobilities as high as 0.04 cm² V⁻¹ s⁻¹ were prepared.

Linear dichroic properties of polyethylene films containing a dispersion of terthiophene-based chromophore molecules are investigated by using a scanning near-field optical microscope (SNOM). The polarization-modulation technique implemented in our SNOM provides quantitative information on the dichroic ratio of the samples with sub-wavelength space resolution. Optically active domains are identified and their morphology is analysed as a function of the film fabrication parameters, e.g., the drawing ratio and the kind of dispersed chromophore mixture. These investigations complement conventional polarimetry analysis by adding nanometre-scale information on the spatial distribution of the chromophore molecules and their mutual alignment with the host polymer chains.

In this work, the influence of downscaling bulk MOSFETs below the 100 nm range on their static and dynamic behaviour is analysed by means of a particle-based Monte Carlo simulator. Internal transport conditions are investigated throughout the extensive information provided by numerical simulations (electric fields, concentration, velocity and energy of carriers, energy bands, etc), and a physical interpretation is given to the dynamic behaviour observed. Results show that even when the most favourable downscaling conditions are considered (that is, following the constant field scaling rules), significant two-dimensional electrostatic effects, together with reduced gate control, lead to the degradation of important figures of merit such as gate-to-source capacitance, transconductance or maximum oscillation frequency. Conventional MOSFET geometries are therefore near to a limit when reaching gate lengths of 50 nm, and the use of alternative solutions is necessary in order to maintain the proper electrostatic behaviour of a 'well-tempered' transistor.

In the present work we report on the optimization of MBE growth conditions and design of metamorphic In(Al)(Ga)As/GaAs heterostructures. This results in a strong decrease in the density of threading dislocations in the upper (active) layers and the improvement of surface morphology. Room-temperature mobility in metamorphic modulation-doped InGaAs/InAlAs heterostructures was 8100 cm² V⁻¹ s⁻¹, which is comparable to that of InP-based structures and noticeably superior to pseudomorphic GaAs-based structures.

InAs quantum dots formed in a metamorphic InGaAs matrix on a GaAs substrate were used for lasers with promising characteristics (emitting wavelengths of 1.46 µm, with threshold current densities of 1.4 kA cm⁻²).

The influence of thermal activation on the motion of a nanotip sliding on a flat surface is discussed. In a dry environment thermal vibrations may induce the tip to jump from an equilibrium position to the next one along its path. This effect leads to a logarithmic increase of friction with the sliding velocity at very low speeds (v<10 µm s⁻¹). At higher speeds thermal activation plays a minor role, and the friction versus velocity curve ends with a plateau. A new analytical formula is discussed, which explains both the increase and the stabilization of friction with velocity. In a humid environment, the situation is complicated by water capillaries, which form between tip and surface, if this is rough. These bridges act as an obstacle for thermally activated jumps. Depending on the wettability of the surface, different behaviours are observed.

Monodispersed spherical Co₈₀Ni₂₀ nanoparticles with diameters between 10 and 540 nm have been prepared by boiling liquid reduction of cobalt and nickel salts through a heterogeneous nucleation process (polyol process). All samples exhibit a core/shell structure with a ferromagnetic core surrounded by a surface layer consisting of antiferromagnetic oxides and organic matter. The ferromagnetic/antiferromagnetic interface, which arises from the core/shell structure, induces an exchange anisotropy at low temperature when the samples are subjected to a field cooling process or to a zero field cooling process starting from a remanent state. The shift of the hysteresis loops (exchange bias) and the increment of the coercive field are consequences of the presence of this magnetic anisotropy. The effect of the particle size, the temperature and the cooling process on the magnetic behaviour of the samples has been analysed.

The lower bound of the energy required to change the state of an electron waveguide Y-branch switch is not thermally limited, and the theoretical limit is orders of magnitude lower than the energy cost of information erasure. Thus as the power dissipation due to information erasure can be avoided by the use of logically reversible gates, such gates based on electron waveguide Y-branch switches promise circuits with extremely low power dissipation. In this paper, reversible logic based on electron waveguide Y-branch switches is proposed and discussed.

Langmuir monolayers of diacetylene lipids with a cytosinyl head-group (PDC), mixed with an alcohol derivative of the same lipid (PDOH), were formed on water and on guanosine solution, and were UV polymerized in situ. Brewster angle microscopy (BAM), atomic force microscopy (AFM) and visible light absorption spectroscopy were performed on the monolayers. It was found that the optimal ratio for the lipid mixtures is approximately 2:1 PDC/PDOH. For lipid mixtures with different ratios, the film decomposes into phases of the preferred composition and a second phase enriched with the excess compound. The stable mixed phase exhibits a distinct striated appearance along the polymer linear direction. Films formed on guanosine-containing subphase have similar phases; however, the stable striated phase appears more developed in the direction perpendicular to the polymer chains. A specific base-pair formation at the air–solution interface between the diacetylene monolayer and the free complementary nucleoside in the solution is suggested.

Highly oriented arrays of lead sulfide (PbS) nanocrystals have been prepared in ambient conditions by the exposure of crystalline polydiacetylene (PDA) Langmuir films to H₂S gas at the air–PbCl₂ solution interface. The nanocrystals were studied by transmission electron microscopy (TEM), electron diffraction (ED), dark field TEM imaging (DF), high resolution TEM (HRTEM) and fast Fourier transform (FFT) analysis of HRTEM images. Three distinct, coexisting orientations were observed with typical nanocrystal morphologies corresponding to each orientation. The direction was aligned parallel to the PDA linear direction for all orientations, indicating a template-directed growth for the PbS/PDA system.

Spinel ferrites (iron cobalt oxide) were prepared by the microemulsion method at different temperatures and sodium dodecyl sulphate (SDS) surfactant concentrations. Subsequently, a quantity of the different samples were coated with an intrinsically conducting polymer (ICP) shell of polypyrrole. The polymer shell was synthesized by a chemical route after the ferrites particle production. By combining in a single material the electrical conductivity of ICPs and the magnetic properties of nanopowder ferrites, new multifunctional materials have been developed. Different CoFe₂O₄ grain size particles were obtained ranging from 3 to 30 nm, as determined by x-ray diffraction (XRD). Particles with grain size below a critical size exhibit a superparamagnetic behaviour. These superparamagnetic particles, without and with a conducting polymer shell, were analysed by transmission electron microscopy (TEM) to find the grains' morphology and the growth evolution of the polypyrrole shell in the ferrites grains. The electrical conductivity of the nanocomposite was measured by the four points probe method, showing values of 120 ± 4 S cm⁻¹ at ambient temperature. The behaviour of the magnetization and the coercivity with temperature, from nearly 0 K to the ambient, were measured in a vibrating sample magnetometer (VSM).